The solid variant of the aneurysmal bone cyst (ABC) is a rare non-neoplastic and reactive bone lesion. It differs from the classical type of ABC in certain aspects.
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Epidemiology
The solid variant of ABC has an incidence of ~5% (range 3.4-7.5%) and is found to have slight female predilection (1.5:1) 1,2.
Pathology
histologically, the solid variant of the ABC and giant cell reparative granuloma are similar in appearance and therefore, these terms are interchangeably used 4,5
it is similar histopathologically to the solid portion of a classical ABC
more than one-third of lesions diagnosed as solid ABCs are non-aneurysmal
Location
giant cell reparative granuloma and the solid variant of ABCs have primarily been seen in the craniofacial and small tubular bones of the hands and feet 1-6
30% cases were found in upper extremities 1-6
in a long bone, no location is exempt; the most commonly seen location is meta-diaphyseal but lesions have also been reported at the end of long bones, within the cortex, and on the surface 1-6
Radiographic features
Plain radiograph and CT
an expansile, eccentric lytic lesion with or without a cortical shell, located in the metadiaphyseal region of a long bone. (In contrast to classical aneurysmal bone cysts, it is not epi-metaphyseal and in 30% of cases it appears non-expansile unlike the classical soap bubble appearance of ABC)
variable degree of mineralization
MRI
T1: iso to hyperintense with respect to muscles with a solid component seen
-
T2
heterogeneously hyperintense solid component
blood fluid levels can be seen in cystic components (mimicking classical secondary aneurysmal bone cysts) but less often seen
bone marrow and soft tissue edema is seen in 50% of cases (distinguishing feature from classical aneurysmal bone cysts)
due to the variable degree of mineralization, few T2 hypointense areas can be seen
T1 C+: solid enhancement seen (in contrast to smooth septal and rim enhancement seen in classical aneurysmal bone cysts)
Treatment and prognosis
Treatment includes cauterisation of the lesion.
Differential diagnosis
classical ABC (epi-metaphyseal location with no soft tissue and bone marrow edema on MRI)
telangiectatic osteosarcoma: 50% of cases show osteoid mineralization. Endosteal scalloping and aggressive bone destruction with a wide zone of transition favors telangiectatic osteosarcoma over sABC on radiographs. Septal enhancement on post-contrast MRI again favors it
giant cell tumor is preferred diagnosis over rarely reported sABC, if the lesion is seen at end of long bones